1. Field of the Invention
The present invention relates to a semiconductor light-emitting material and a light emitting device.
2. Description of the Related Art
Silicon, which is a typical electronic material supporting electronics, has been considered unsuitable for application in the optical field. This is because silicon is of an indirect transition type and the conduction band and valence band of silicon are p-orbital (odd function), making silicon unsuitable for application in the optical field. The essential points of these reasons are said to reside in the preservation of the wave number not being established and the matrix component of the dipole moment being made very small.
A nano-particle technology and a superlattice technology are known as prior arts of the silicon light emitting device. The emission mechanism in the nano-particle technology is considered to be based on the “confinement effect”. The emission mechanism in the superlattice technology is considered to be based on the “zone-folding effect”. These two effects have substantially the same function and are intended to make the band structure of silicon into direct transition type. However, both the conduction band and the valence band contain mainly the p-orbital component. It is theoretically impossible for the silicon light emitting device based on these techniques to emit light at high efficiency and high speed. To be more specific, each of the “confinement effect” and the “zone-folding effect” simply brings about a pseudo-dipole transition. Therefore, it is not expected intense emission using only any one of the effects. Therefore, it has been impossible for the conventional silicon light-emitting device to exhibit sufficient characteristics and to be put into a practical use.
In the case of utilizing the confinement effect which has been considered most hopeful, a serious problem resides in that it is unavoidable to introduce a high electronic barrier. It is difficult to inject a current over the high electronic barrier, so that the electroluminescence (EL) by the current injection method is inhibited.
As described above, conventional research has been concentrated on the idea of converting silicon into a substance of a direct transition type. However, practically required is a band modulating technology in which one of the conduction band and the valence band is made to be an s-orbital band (even function), with the other is left to be a p-orbital band (odd function). Further, required is a method that permits the emission by the current injection without introducing any electronic barrier.
Conventional techniques relating to the silicon light-emitting device other than those based on the confinement effect and the zone-folding effect referred to above are also known to the art as follows.
Rompa et al. found from band calculation of GaAs that the X-point energy is raised in the GaAs having He introduced into the interstitial site of GaAs, as described in Phys. Rev. Lett., 52, 675 (1984). This is called a filled tetrahedral (FT) semiconductor. Since GaAs is a direct transition type semiconductor that permits emission at high speed, a prominent effect is not produced even if the X-point is raised, though the possibility of modulating the X-point is pointed out. However, the document does not show at all to the modulation of the Γ-point relating to the improvement in the emission efficiency of the indirect semiconductor.
It is reported by Burger et al. in Phys. Rev. Lett., 52, 1645 (1984) that the FT semiconductor prepared by ion-implantation of rare gas such as He, Ne, Ar, Kr and Xe into a silicon wafer generates PL emission in the energy level of approximately 1 eV. However, the PL emission ceases to be generated if the wafer is annealed at a temperature of several hundred degrees. It follows that the FT semiconductor containing a rare gas is expected to be low in thermal stability and, thus, to be impracticable.
Pickett et al. teach change in the band structure in the case of applying a uniaxial stress to carbon, as described in SPIE vol. 877 Micro-Optoelectronic Materials 64 (1988). However, even if carbon is converted into a material of a direct transition type, a light emitting device having high efficiency and operated at high speed cannot be expected if the amount of the s-orbital component of the conduction band remains to be small. Further, the discussion given in the document covers the case where uniaxial strain as high as 4% is introduced into a very hard carbon, which is impossible to achieve actually. As a matter of fact, any experiment covering the particular situation is not shown.
As described above, conventional techniques have a serious problem that it is impossible to modulate sufficiently the band structure of an indirect semiconductor, e.g., silicon. Since an orbital component cannot be changed, it is impossible to use effectively a matrix component of dipole moment. Also, in the case of utilizing the confinement effect, the electrons will be confined by a high electronic barrier, leading unavoidably to a problem that current injection is made difficult.